All cellular systems are divided into cells, served by one specific base station. Each base station may serve more than one cell. The important point from a positioning and navigation perspective is that the cell where a specific UE (User Equipment=terminal=cellular phone) is located is known in the cellular system. Hence, after determination of the geographical area covered by a specific cell, it can be stated that the UE is located somewhere within said geographical area, as long as it is connected and the reported cell identity of the serving cell is equal to the cell identity of the particular geographical area.
In several systems, among these the WCDMA (Wideband Code Division Multiple Access) system, the preferred representation of the geographical extension of the cell is given by the so called cell polygon format [1]. The geographical extension and position of a cell is described by 3-15 corners of a closed polygon, which does not intersect itself. The format is two-dimensional and the corners are determined as pairs of longitudes and latitudes in the WGS84 geographical reference system, see [1] for details.
The cell identity positioning method operates as follows (the description is done for the WCDMA cellular system, assuming that the positioning operates over the RANAP interface [2]. The procedures are similar for GSM and control plane LTE.
1) The message LOCATION REPORTING CONTROL is received in a SRNC (Serving Radio Network Controller) over the RANAP interface [2].
2) The quality of service parameters (most importantly accuracy and response time) of the LOCATION REPORTING CONTROL message is such that the RNC selects the cell identity positioning method.
3) The SRNC determines the serving cell identity of the positioned UE (special procedures may apply in case the UE is in soft(er) handover with multiple base stations), and retrieves a pre-stored polygon that represents the extension of the serving cell.
4) The SRNC sends the resulting cell polygon back to the core network over the RANAP interface [2], using the cell polygon format in a LOCATION REPORT message.
As stated above, the preferred representation of the geographical extension of the cell is given by the cell polygon format [1]. FIG. 1 represents an example of a cell polygon with corners A-E. The NodeB (RBS, Radio Base Station) is normally located close to one of the corners of the cell polygon said NodeB serves. The 3GPP Polygon message IE (Information Element) in FIG. 2 is present in the LOCATION REPORT message that is returned to the core network over the RANAP interface after a successful cell identity positioning.
It should be noted that due to the complexity of the radio propagation the cell polygon format is only an approximation of the extension of the true cell. The selection of the polygon format is dictated by the need to have a reasonably flexible geographical representation format, taking e.g. computation complexities and reporting bandwidths into account.
Since the polygon format approximates the cell extension, the polygon is normally pre-determined in the cell-planning tool to represent the cell extension with a certain confidence. The confidence is the probability that the terminal is actually located within the reported region, in this case bounded by the cell polygon.
So-called Assisted GPS (A-GPS) positioning is an enhancement of the global positioning system (GPS). An example of an A-GPS positioning system is displayed in FIG. 3. There GPS reference receivers attached to a cellular communication system collect assistance data that, when transmitted to GPS receivers in terminals connected to the cellular communication system, enhance the performance of the GPS terminal receivers. Typically, A-GPS accuracy can become as good as 10 meters also without differential operation. The accuracy becomes worse in dense urban areas and indoors, where the sensitivity is most often not high enough for detection of the very weak signals from the GPS satellites.
Another positioning approach is provided by so called fingerprinting positioning [3], which operates by creating a radio fingerprint for each point of a fine coordinate grid that covers the Radio Access Network (RAN). The fingerprint may e.g. consist of
i) The cell Ids that are detected by the terminal, in each grid point.
ii) Quantized path loss or signal strength measurements, wrt multiple NodeBs, performed by the terminal, in each grid point. Note, an associated ID of the NodeB may also be needed.
iii) Quantized round trip time (RTT), in each grid point. Note, an associated ID of the NodeB may also be needed.
iv) Radio connection information like the radio access bearer (RAB).
Whenever a position request arrives to the positioning method, a radio fingerprint is first measured, after which the corresponding grid point is looked up and reported. This of course requires that the point is unique. The database of fingerprinted positions (the radio map) can be generated in several ways. A first alternative would be to perform an extensive surveying operation that performs fingerprinting radio measurements repeatedly for all coordinate grid points of the RAN. The disadvantages of this approach include:
The surveying required becomes substantial also for small cellular networks.
The radio fingerprints are in some instants (e.g. signal strength and path loss) sensitive to the orientation of the terminal, a fact that is particularly troublesome for handheld terminals. For fine grids, the accuracies of the fingerprinted positions therefore become highly uncertain, which is unfortunately seldom reflected in the accuracy of the reported geographical result.
Another approach is to replace the fine grid by high precision position measurements of opportunity, and to provide fingerprinting radio measurements for said points. This avoids the above drawbacks, however
Algorithms for clustering of high precision position measurements of opportunity needs to be defined.
Algorithms for computation of geographical descriptions of the clusters need to be defined.
The above two problems are solved by prior art in relation to the AECID positioning method, see [3].
One particular form of fingerprinting is the so-called AECID fingerprinting positioning method, one of the first steps of which is to collect tagged high precision reference measurements, e.g. obtained by A-GPS measurements and tagging with measured radio conditions, in clusters where all high precision measurements have the same tag. This creates clusters of tagged high precision measurements, like the one shown in FIG. 4. The picture shows a simulated cluster of high precision reference positions. Each dot represents one high precision (A-GPS) measurement. All high precision measurements have the same tag.
Previous patent applications and [3] describe how a polygon can be computed, to describe the boundary of the cluster of high precision measurements shown in FIG. 4.